Here we show how to design organic redox-active solutions for use in redox-flow batteries, with an emphasis on attaining high volumetric capacity electrodes that minimize active-material crossover through the flow cell’s membrane. Specifically, we advance oligoethylene oxides as versatile core motifs that grant access to liquid redox-active oligomers having infinite miscibility with organic electrolytes. The resulting solutions exhibit order-of-magnitude increases in volumetric capacity and obviate deleterious effects on redox stability. The design is broadly applicable, allowing both low potential and high potential redox centers to be appended to these core motifs, as demonstrated by benzofurazan, nitrobenzene, 2,2,6,6-tetramethylpiperidin-1-yl)­oxyl, and 2,5-di-tert-butyl-1-methoxy-4-(2′-methoxy)­benzene pendants, whose reduction potentials range from −1.87 to 0.76 V vs Ag/Ag+ in acetonitrile. Notably, the oligoethylene oxide scaffold minimizes membrane crossover relative to redox-active small molecules, while also providing mass- and electron-transfer kinetic advantages over other macromolecular architectures. These characteristics collectively point toward new opportunities in grid-scale energy storage using all-organic redox-flow batteries.

Redox active colloids (RACs) are dispersible, cross-linked polymeric materials that incorporate a high concentration of redox-active motifs, making them attractive for next-generation size-exclusion redox flow batteries. In order to tap into their full potential for energy storage, it is essential to understand their internal charge mobility, capacity, and cyclability. Here we focus on using a combined suite of Raman spectroscopy and scanning electrochemical microscopy (SECM) tools for evaluating three important parameters that govern charge storage in viologen-RACs: their intraparticle redox active concentration, their reduction/oxidation mechanism, and their charge transfer rate. We addressed RACs using SECM imaging and single-particle experiments, from which the intraparticle diffusion and concentration parameters were elucidated. By using Raman spectroscopy coupled to surface interrogation SECM, we further evaluated their reversible redox properties within monolayer films of 80- and 135-nm-sized RACs. Most notably we have confirmed that the concentration and redox mechanisms are essentially unchanged when varying the RAC size. As expected, we see that larger particles inherently require longer times for electrolysis independent of the methodology used for their study. Our simulations further verify the internal concentration of RACs and suggest that their porosity enables solution redox active mediators to penetrate and titrate charge in their interior. The combined methodology presented here sets an important analytical precedent in decoupling the charge storage properties of new bulk materials for polymer batteries starting from probing low-dimensional assemblies and single particles using nano- and spectroelectrochemical approaches.

The importance of the active material structure and the interface/interphase between the electrode and electrolyte in enhancing the electrochemical performance of sodium and potassium ion batteries.

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Nanoparticle metal oxide photocatalysts are attractive because of their increased reactivity and ease of processing into versatile electrode formats; however, their preparation is cumbersome. We report on the rapid bulk synthesis of photocatalytic nanoparticles with homogeneous shape and size via the cathodic corrosion method, a simple electrochemical approach applied for the first time to the versatile preparation of complex metal oxides. Nanoparticles consisting of tungsten oxide (HWO) nanoplates, titanium oxide (TiO) nanowires, and symmetric star-shaped bismuth vanadate (BiVO) were prepared conveniently using tungsten, titanium, and vanadium wires as a starting material. Each of the particles were extremely rapid to produce, taking only 2-3 min to etch 2.5 mm of metal wire into a colloidal dispersion of photoactive materials. All crystalline HWO and BiVO particles and amorphous TiO were photoelectrochemically active toward the water oxidation reaction. Additionally, the BiVO particles showed enhanced photocurrent in the visible region toward the oxidation of a sacrificial sulfite reagent. This synthetic method provides an inexpensive alternative to conventional fabrication techniques and is potentially applicable to a wide variety of metal oxides, making the rapid fabrication of active photocatalysts with controlled crystallinity more efficient.